JPH0227407B2 - YOSETSUSEINISUGURETAKOKYODOKONOSEIZOHOHO - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing high-strength steel,
This invention relates to steel useful as a welded structural material for pressure vessels, bridges, construction machinery, etc., which require high strength and good weldability. (Prior Art) Conventionally, high-strength steel for the above-mentioned uses has been manufactured by off-line quenching and tempering heat treatment methods. However, in order to obtain high strength, many types of alloying elements are added to these steels, which results in high manufacturing costs and requires preheating to a high temperature during welding to prevent weld cracking. As a conventionally proposed method, a method described in Japanese Patent Publication No. 41-2763 is known. This is due to Mo-Nb system precipitates due to the coexistence of Mo and Nb.
It uses precipitation hardening to impart high strength.
All of these are based on the premise that they are manufactured off-line through normal quenching and tempering heat treatment. In this case, the quenching temperature is around 900℃, so in order to achieve sufficient precipitation hardening, many
The addition of Nb and Mo is required, which increases costs, and there is a problem with weldability, particularly weld cracking, which requires preheating at a high temperature during welding, and there is a strong desire for improvement. (Problems to be Solved by the Invention) The present invention has a high degree of toughness and good weldability necessary for such uses, especially weld cracking resistance, and has a high tensile strength of 80 Kgf/mm ² or more. The objective is to provide a method for inexpensively manufacturing steel having a thickness of over 50 mm. (Means for Solving the Problems) In order to advantageously achieve the above object, the present inventors have developed a steel with a low carbon equivalent and high strength and toughness of 80 Kgf/mm ² or more. As a result of various studies on manufacturing methods, we found that by on-line quenching a steel that combines Nb, Mo, B, and low N, and then tempering it, a high-toughness, high-strength steel with a low carbon equivalent and extremely excellent properties can be obtained. I found out that. The present invention was constructed based on such knowledge, and its gist is that C: 0.04 to 0.11%, Si: 0.1 to 1.0%,
Mn: 0.50-2.00%, Mo: 0.10-1.0%, Nb:
0.005~0.05%, B: 0.0003~0.01%, Al: 0.005~
0.1%, N: 0.0060% or less, and if necessary, Cr: 1% or less, Ni: 1% or less,
A strength-improving element group consisting of Cu: 1% or less, V: 0.1% or less, and one or more of the strength-improving element group of Ca: 0.01% or less,
Alternatively, one or more elements are selected from the group of strength-improving elements, Ca is added thereto, and the remainder is Fe and unavoidable impurities, and the steel is heated and rolled at a heating temperature of 1000 to 1270°C and a rolling end temperature of 800 to 1100°C, A method for producing high-strength steel with excellent weldability, characterized in that immediately after rolling, the steel is rapidly cooled to 200°C or less and then subjected to tempering heat treatment at Ac ₁ or less. (Action of the invention) As a result of various experiments and studies, the present inventors found that
By online quenching and then tempering a suitable Nb-Mo-B-N component system, it is possible to dramatically utilize the precipitation hardening of Mo-Nb precipitates, and improve the hardenability of B and Mo. We have found that it is possible to significantly enhance the improvement effect. In other words, the present invention is capable of producing significant precipitation hardening that could not previously be expected during tempering after on-line quenching with a small amount of Nb due to solid solution of Nb and Mo during online heating, and by aiming to reduce the amount of Nb. This is based on two findings: that B greatly improves the hardenability of B, and that combined with the hardenability improving effect of Mo, the hardenability is significantly improved. This method makes it possible to manufacture steel with a tensile strength of 80 Kgf/mm ² or more, which has good low-temperature toughness and weldability with relatively few chemical components, and is especially suitable for use with plate thicknesses exceeding 50 mm. It is also possible to produce meat materials. Next, the reason for limiting the chemical composition of the target steel in the present invention will be described. 0.04% or more of C is required to obtain high-strength steel, and the strength increases as the content increases.
Since low-temperature toughness and weld cracking resistance deteriorate, the content is set to 0.04 to 0.11%. Si usually exists as a deoxidizing element, but 0.1% or more is required to improve strength. However, if it exceeds 1.0%, the low-temperature toughness decreases significantly, so the upper limit is 1.0%.
shall be. Mn is required at 0.50% or more to obtain high strength, but if it exceeds 2.0%, low-temperature toughness and weldability will be impaired, so the content should be 0.50-2.0%. Mo is useful for improving strength and low-temperature toughness. In Although,
If it is less than 0.1%, the effect is small. On the other hand, if it exceeds 1%, the strength becomes too high, leading to a decrease in low-temperature toughness and making it expensive, so it is set at 0.1 to 1.0%. It is preferably 0.25 to 0.6%. Nb coexists with Mo and is useful for precipitation hardening during tempering, and also makes it possible to improve the hardenability of B in combination with a low N content, but if it is less than 0.005%, it is ineffective. Also
If it exceeds 0.05%, the cost will increase and weldability will be impaired, so the content should be 0.005 to 0.05%. Preferably 0.01
~0.025% is good. 0.005% or more of Al is necessary for deoxidizing steel, but if it exceeds 0.1%, it will inhibit the cleanliness of steel.
The upper limit is 0.1%. B is generally useful for increasing hardenability, and in particular in the steel of the present invention, the combined effects of Nb addition, low N content, and Mo addition can significantly improve hardenability. 0.0003% or more is required. However, if added in large amounts, weldability will be impaired, so the upper limit is set at 0.01%. Although N is generally an unavoidable element and an element that inhibits the hardenability of B, the upper limit at which a small amount of Nb can improve hardenability is 0.006%.
The content is preferably 0.004% or less. In the present invention, in addition to the above-mentioned basic components, one or more of the following elements can be selectively included depending on the required characteristics of the steel. Cr, Ni, Cu, and V have the uniform effect of improving the strength of steel, and are included as necessary, but Cr: 1.0%, Ni: 1.0%,
Even if the content exceeds the upper limits of Cu: 1.0% and V: 0.1%, the above strength-improving elements will be too expensive and have a negative effect of inhibiting weldability, which will have the opposite effect. Define upper limits for each component of the group. Ca is added during steelmaking to improve the deoxidation of steel.
This is useful for reducing inclusions and controlling the morphology of sulfide-based inclusions to improve low-temperature toughness, but if present in large quantities in steel, harmful non-metallic inclusions will be produced, and conversely, they will reduce low-temperature toughness. To prevent this, the content should be 0.01% or less. Next, P and S, which are unavoidably contained as impurities, are not particularly limited, but in order to stabilize the material quality through the cleanliness of the steel, the less the better.From this perspective, P should be 0.020% or less, and S should be 0.010%
The following is desirable. Next, the reasons for limiting the heating-rolling-heat treatment conditions for steel having the above composition will be described. The heating temperature needs to be 1000°C or higher to form a solid solution of Nb, but if it exceeds 1270°C, the coarsening of γ grains becomes significant, so the upper limit is set at 1270°C. Rolling is carried out immediately after heating and extraction of the slab. Therefore, the rolling start temperature is almost determined by the slab heating temperature and plate thickness. Therefore, the rolling end temperature was set to 1100.
If you try to exceed ℃, you will have to take measures to suppress the temperature drop, which will increase the cost, so the upper limit should be set.
The temperature shall be 1100℃. Further, the rolling end temperature is set to 800° C. or higher because the lower the temperature, the lower the hardenability, and the lower the low-temperature toughness after tempering. Next, immediately after rolling, quenching is performed, but if the cooling start temperature becomes low, hardenability decreases, so quenching from 800° C. or higher is preferable. This rapid cooling is performed online by supplying a cooling medium such as water or mist to the front and back surfaces of the steel plate. The temperature after quenching is 200℃ because it is difficult to obtain a completely hardened structure if the temperature is high.
is the upper limit. After the above treatment, a tempering heat treatment is performed, and since tempering in the ferrite region is essential for obtaining good low-temperature toughness, the upper limit temperature is set to Ac ₁ temperature. (Example) Next, Examples will be given together with Comparative Examples. Using steel having the chemical composition shown in Table 1,
The heating-rolling-heat treatment shown in the table was performed. The mechanical properties and weld cracking resistance of the obtained steel plate are shown in Table 2.

【table】

【table】

[Table] Note 1: Rapid cooling start temperature is not much different from rolling end temperature Note 1: DQT: Direct quenching-tempering, QT: Off-line quenching-tempering As is clear from Table 2, in the case of the examples of the present invention, In addition to high strength of over 80Kgf/ ^mm2 ,
It exhibits high toughness of -60℃ or less for a 25mm plate thickness, and -50℃ or less for a 55mm plate thickness, and has extremely good weld cracking resistance with a Y crack stop temperature of room temperature, which is one criterion for weld crackability. We were able to manufacture a thick steel plate that is easy to use for users with In particular, Examples A and B contain only the basic components, and Examples C, D, E, and F contain Cu, Cr,
This is an example containing one or more alloying elements of Ni and V, which are strength improving elements. Example G has in addition to the basic components
This is an example containing Ca. Examples H and I are Cu, Cr, Ni, V
This is an example in which Ca is included in addition to the strength-enhancing element. Example B ₂ is a comparative example and the heating temperature was too high, so
Low temperature toughness decreased. Examples J, K, and L are comparative examples. Example J does not contain B and has been subjected to DQT treatment, but it is somewhat lacking in the strength of 80kg steel and has low low temperature toughness. Example K is an example in which the amount of N is high, but the strength is still low and the low temperature toughness is also low. Example L is an example with a high C content, and is an example of conventional off-line quenching and tempering. Although the strength and toughness are almost good, the Y cracking stop temperature is 125°C, and the welding cracking property is poor. (Effects of the invention) As described above, the present invention uses Nb-Mo-B-low N steel and uses a heating-rolling-online quenching-tempering process to maximize the effective precipitation of solid solution Nb and Mo during heating. In addition to being used for low N and Mo, Nb
The hardenability of B has been thoroughly improved through the combined addition of B, giving it excellent hardenability.
It is said that it is possible to produce high-toughness, high-strength steel at an extremely low cost because the Ni, which was added in large amounts in kilo-grade steel, is largely omitted, and a small amount of alloying elements such as Nb and Mo are added, and offline quenching heat treatment can be omitted. effective.

Claims (1)

[Claims] 1 C: 0.04 to 0.11%, Si: 0.1 to 1.0 by weight
%, Mn: 0.50-2.00%, Mo: 0.10-1.0%,
Nb: 0.005-0.05%, B: 0.0003-0.01%, Al:
A steel whose basic components are 0.005 to 0.1%, N: 0.0060% or less, and the balance is Fe and unavoidable impurities.
Heating temperature 1000~1270℃, rolling end temperature 800~1100
1. A method for producing high-strength steel with excellent weldability, characterized by hot rolling at a temperature of 200°C, quenching immediately after rolling to 200°C or less, and then subjecting it to tempering heat treatment at an Ac of ₁ point or less. 2 C: 0.04-0.11%, Si: 0.1-1.0 in weight%
%, Mn: 0.50-2.00%, Mo: 0.10-1.0%,
Nb: 0.005-0.05%, B: 0.0003-0.01%, Al:
The basic components are 0.005 to 0.1%, N: 0.0060% or less, and further Cr: 1% or less, Ni: 1% or less,
Containing one or more of the strength improving element group consisting of Cu: 1% or less and V: 0.1% or less,
Steel consisting of the balance Fe and unavoidable impurities is heated and rolled under the conditions of a heating temperature of 1000 to 1270℃ and a rolling finish temperature of 800 to 1100℃, and immediately after rolling, it is rapidly cooled to 200℃ or less, and then subjected to tempering heat treatment at Ac ₁ point or less. A method for producing high-strength steel with excellent weldability. 3 C: 0.04-0.11%, Si: 0.1-1.0 in weight%
%, Mn: 0.50-2.00%, Mo: 0.10-1.0%,
Nb: 0.005-0.05%, B: 0.0003-0.01%, Al:
The basic components are 0.005 to 0.1%, N: 0.0060% or less, and Ca: 0.01% or less, and the balance
Steel consisting of Fe and unavoidable impurities is heated to
High weldability characterized by hot rolling at 1000-1270℃ and rolling end temperature 800-1100℃, rapid cooling to 200℃ or less immediately after rolling, and then tempering heat treatment at Ac ₁ point or less. Method of manufacturing strength steel. 4 C: 0.04-0.11%, Si: 0.1-1.0 in weight%
%, Mn: 0.50-2.00%, Mo: 0.10-1.0%,
Nb: 0.005-0.05%, B: 0.0003-0.01%, Al:
The basic components are 0.005 to 0.1%, N: 0.0060% or less, and further Cr: 1% or less, Ni: 1% or less,
One or more of the strength improving element group consisting of Cu: 1% or less, V: 0.1% or less, and Ca: 0.01
% or less, with the remainder being Fe and unavoidable impurities, is heated and rolled at a heating temperature of 1000 to 1270°C and a rolling end temperature of 800 to 1100°C, and immediately after rolling, it is rapidly cooled to 200°C or less, and then Ac ₁ A method for producing high-strength steel with excellent weldability, which is characterized by subjecting it to tempering heat treatment at a temperature below 100%.